1

Advanced Materials for Rechargeable Lithium-Sulfur Batteries

Yongzhu Fu

Department of Mechanical Engineering Richard G. Lugar Center for Renewable Energy

Indiana University – Purdue University Indianapolis

yongfu@iupui.edu

May 6, 2014

High Energy Density Li-ion Batteries

• Higher voltage & energy density

• Compact, light weight

• Long cycle life

• Wider temp. range (-40 to 70 °C)

Load/charger

Electrolyte

e-e-

CathodeAnode

Li+

Li+

Charge

Discharge

LixC6 Li1-xCoO2

2

Electrode Materials for Li-ion Batteries

3

MO2 Li MO2 Li MO2 Li MO2 Li[Li,Mn,Ni,Co]O2

Manthiram, J. Phys. Chem. Lett. 2011, 2, 176.

Layered

Spinel

Olivine

Current Status and Challenges

4

Electrode Material

Cell Voltage

(V)

Capacity (Ah/kg)

Specific Energy (Wh/kg)

Advantages and disadvantages

Layered LiCoO2

Cathode

~ 4 140 560 Expensive; toxic; safety concerns; only 50 % theoretical capacity utilized; 2D structure; used in portable devices

Spinel LiMn2O4

Cathode

~ 4 120 480 Inexpensive; environmentally benign; better safety; 3D structure; high rate capability; severe capacity fade at elevated temperatures (55 oC)

Olivine LiFePO4

Cathode

~ 3.5 160 560 Inexpensive; environmentally benign; covalently bonded PO4 groups offer excellent safety; 1D structure; low Li+ and electronic conductivity; controlled manufacturing & high processing cost

Carbon

Anode

~ 0.1 370 _ Inexpensive; environmentally benign; low operating potential maximizes cell voltage; SEI layer and lithium plating lead to safety concerns

Sulfur – An Abundant Cathode Material

5

Lithium-sulfur battery Elemental sulfur

Capacity 1,672 mAh/g Density 2.07 g/cm3

Gravimetric energy density 2,500 Wh/kg Equivalent weight 32.0 g/mol

Volumetric energy density 2,800 Wh/L Melting point 115.21 oC

Average operating voltage 2.15 V Electrical resistivity 2 × 1015 Ωm

S + 2Li+ + 2e- ↔ Li2S

2 electron reaction

Challenges with Sulfur Cathode

6

Bruce et al., Nat. Mater. 2012, 11, 19.

• Poor rechargeability and limited rate capability owing to the insulating nature of sulfur and the solid reduction products (Li2S and Li2S2)

• Fast capacity fade owing to the generation of soluble polysulfides Li2Sn

• Shuttle of polysulfides between anode and cathode results in unlimited charging, low Coulombic efficiency, and poor cycle life

Mikhaylik et al., 216th ECS Meeting, October, 2009.

Advanced Cathode Materials for Li-S Batteries

0 10 20 30 40 500

200

400

600

800

1000

1200

Spec

ific

capa

city

(mAh

/g)

Charge (C/5) Discharge (C/5)

Coulombic efficiency (%

)

Cycle number

20

40

60

80

100

J. Phys. Chem. C 2012, 116, 8910 Phys. Chem. Chem. Phys. 2012, 14, 14495 RSC Adv. 2012, 2, 5927 Chem. Mater. 2012, 24, 3081

90100110

0 10 20 30 40 50

0

500

1000

1500

1C 2C 3C

Coul

ombi

c ef

ficie

ncy

(%)

1C 2C 3C

Disc

harg

e ca

paci

ty (m

Ah/g

- su

lfur)

Cycle Number

Angew. Chem. Int. Ed. 2013, 52, 6930 J. Am. Chem. Soc. 2013, 135, 18044 Nat. Commun. 2013, 4, 2985 Adv. Energy Mater. 2014, 4, 1300655

Li

MWCNT paper

Polysulfide (e.g., Li2S6) catholyte

Load/Charger

+-e-e-

chargedischarge

Li+

Li+

Celgard separator 0 10 20 30 40 500

400

800

1200

1600

Capa

city

(mAh

g-1)

Cycle number

C/10 C/5 C/2

Li

Li

Li2S6

Li2S

Li

Li

Li2S6 + 10Li 6Li2S3.56 Å

7

Li/Dissolved Polysulfide Batteries

8

S8

Li2S8

Li2S6

Li2S4

Li2S2

Li2S

Soluble

Insoluble

Insoluble

Rauh et al. J. Electrochem. Soc. 1979, 126, 523.

• Soluble polysulfides are liquid cathode materials which can provide high specific capacities due to their high solubility, up to 10 M of sulfur in THF

• Very limited success has been achieved due to poor electrodes Zhang, Read, J. Power Sources 2012, 200, 77.

Cell Configuration

9

Li

MWCNT paper

Polysulfide (e.g., Li2S6) catholyte

Load/Charger

+-e-e-

chargedischarge

Li+

Li+

Celgard separator

Fu, Su, Manthiram, Angew. Chem. Int. Ed. 2013, 52, 6930.

• Binder-free, self-weaving multi-walled carbon nanotube (MWCNT) electrodes with abundant space for charged/discharged products and electrolyte penetration

Voltage Profile, SEM/TEM, and XRD

10

1 µm

0 5 10 15 20 251.0

1.5

2.0

2.5

3.0

2nd charge

Cell v

olta

ge (V

)

Time (h)

1st charge 1st discharge

167 mA g-1 (C/10)

500 nm

10 20 30 40 50 60 70

43.3o

1st discharge

1st charge

CuKα 2θ (degree)

MWCNTInte

nsity

(a.u

.)

26.3o

• Amorphous insoluble charged/discharged products are fully trapped within the CNT electrodes

XPS and LC Characterization

11

• XPS shows that the charged products are soluble polysulfides and sulfur, while the discharged product is almost completely the end product Li2S

• LC shows that the active material is converted to Li2S that is completed trapped within the carbon nanotube electrode

(a)a

charged

charged, washed discharged

discharged, washed

(c)

blank electrolyte

polysulfide

charged

b

discharged

Electrochemical Performance

12

• The system shows remarkable electrochemical stability

• The system exhibits unprecedented high discharge capacities

-1,600 mAh/g initially at C/10 rate -1,411 mAh/g after 50 cycles

1.8 2.0 2.2 2.4 2.6 2.8 3.0-8-6-4-20246

Curre

nt d

ensit

y (m

A cm

-2)

Voltage (V)

Initial 1st 5th 10th

a

0 500 1000 1500 20001.5

2.0

2.5

3.0

C/2 C/5

Cell v

olta

ge (V

)

Capacity (mAh g-1)

C/10

C/2 C/5 C/10b

0 10 20 30 40 500

400

800

1200

1600

Capa

city

(mAh

g-1)

Cycle number

C/10 C/5 C/2c

Capacity Control Charge

13 Su, Fu, Manthiram, Nat. Commun. 2013, 4, 2985.

Redox Flow Li-S Batteries

14

Manthiram, Fu, Su, Acc. Chem. Res. 2013, 46, 1125.

Li Li2S6+-

Load/Charger

separator

• A half-flow-mode Li-S battery with catholyte circulating at the cathode side like redox flow batteries can be developed

• Control of the discharge cutoff voltages can allow soluble polysulfides to be present in the catholyte, which could lead to long cycle life

Yang, Zheng, Cui, Energy Environ. Sci. 2013, 6, 1552.

Li2S Sandwiched Electrode

15 Fu, Su, Manthiram, Adv. Energy Mater. 2013, 4, 1300655.

Li2S

Li2S powder

Li2S embedded MWCNT electrode

Li+

MWCNT paper

2.0 2.4 2.8 3.2 3.6 4.0-1.5

-1.0

-0.5

0.0

0.5

1.0

Curre

nt d

ensit

y (m

A cm

-2)

Voltage (V)

Initial 1st cycle 2nd cycle 5th cycle 10th cycle

• The nanostructured carbon nanotube electrode provides an ideal environment for electrochemical reactions of Li2S with sufficient ion and electron transport

• An energy barrier exists for large Li2S particles to become active

• After the activation, the cell exhibits excellent electrochemical stability

Voltage Profile and XRD

16

• The voltage profile resembles the CV plot

• After initial activation, the Li2S sandwiched electrode behaves like a conventional sulfur electrode

• XRD results conform that crystallized Li2S is converted to amorphous polysulfides or sulfur after charge, which are transformed back to crystallized Li2S during the following discharge

0 20 40 60 80

2

3

4

Volta

ge (V

)

Time (h)

initial

1st charge

1st discharge

20 30 40 50 60 70 80

(311

)

(220

)

1st discharge

1st chargeInte

nsity

(a.u

.)

CuKα 2θ (degree)

initial

(111

)

Li2S

SEM and Impedance Analysis

17

• SEM reveals the reversible process of Li2S -> polysulfides -> Li2S

• EIS analysis shows an increase of impedance after charge due to the decrease of surface area, and a decrease of impedance after discharge due to the increase of surface area within the electrode

0 100 200 300 400 5000

100

200

300

400

500 Initial 1st charge 1st discharge

-Z'' (

ohm

)

Z' (ohm)

a b c

e

500 µm 20 µm 50 µm

d

Li2S charge discharge

Electrochemical Performance

18

0 20 40 60 80 1000

200

400

600

800

1000

1.0 mg cm-2

2.0 mg cm-2

3.0 mg cm-2

Capa

city

(mAh

g-1

)

Cycle number

C/2

c

0 200 400 600 800 10001.5

2.0

2.5

3.0

1CC/2C/5

Cell v

olta

ge (V

)

Capacity (mAh g-1)

C/10a 0 20 40 60 80 100

0

200

400

600

800

1000

Coulo

mbic

effic

iency

(%)

C/10 C/5 C/2 1C

Capa

city (

mAh

g-1)

Cycle number

0

50

100

150

200

250

b

• The sandwiched electrodes exhibit high capacities, high rate capability, and long cycle life

• High overpotential at high rates is due to the thick electrodes

• The sandwiched electrode can have high Li2S loadings

In Situ Formed Li2S in a Lithiated Graphite Electrode

19 Fu, Zu, Manthiram, J. Am. Chem. Soc. 2013, 135, 18044.

Li

Li

Li2S6

Li2S

Li

Li

Li2S6 + 10Li 6Li2S3.56 Å

0 1 2 3 40.0

0.2

0.4

0.6

0.8

1.0 lithiation

Volta

ge (V

)

Capacity (mAh) Li-CP

Polysulfide (Li2S6) catholyte

100 µm

Li + 6C LiC6

XRD and Electrochemical Behavior

20

10 20 30 40 50 60 70

(004)(002)

2θ (°)2θ (°)

Li2S-CP

Li-CP

2θ (°)

CP

Inte

nsity

(a.u

.)

22 24 26 28 30 40 44 48 52 56 60

0 2 4 6 8 10 12 140

1

2

36d

3d

1dVolta

ge (V

)

Time (hour)

6d, 3d, 1d, as-prepared

as-prepared

0 1 2 3 4 5 6 7 8 90.0

0.5

1.0

1.5

2.0

2.5

2.11 V

2.12 V2.10 V

1.65 V

1.27 V0.97 V

0.71 V0.45 V

Volta

ge (V

)

Time (day)

0.06 V

XPS, SEM, and Electrochemical Performance

21

172 170 168 166 164 162 160 158

cycledLi2S-CP

Li2Sx

(163.2)

Li2S2

(161.7)

Binding energy (eV)

Inte

nsity

(a.u

.)LiCF3SO3 Li2S

(160.1)

Li2S-CPS

S

Li2S-CP

cycled

0 10 20 30 40 500

200

400

600

800

1000

Coul

ombi

c ef

ficie

ncy

(%)

Capa

city

( mAh

g-1)

Cycle number

0

20

40

60

80

100

48.4%

54.9%

Conclusions

22

Li/dissolved polysulfide batteries • Binder-free carbon nanotube electrodes with abundant spaces for

trapping charged/discharged products and electrolyte penetration, leading to unprecedented discharge capacities

• Half-flow mode Li-S batteries could be developed for grid energy or renewable energy storage

Li2S electrodes • Carbon nanotube electrodes provide an electrochemically favorable

environment for ion and electron transport, leading to unprecedented capacities and cyclability of Li2S

• In situ formed Li2S in lithiated graphite is a feasible approach to develop Li2S cathodes, it can be adapted to other lithium-deficient materials

Outlook

23

• Significant improvements have been made with Li-S batteries, but the cycle life is still below the requirement for practical applications.

• Lithium metal anode needs to be improved or alternative anodes need to be developed. Solid-state electrolytes could eliminate shuttle effects.

• To significantly improve Li-S battery performance, novel materials or cell configurations (e.g., coatings on electrodes or interlayers between cathode and separators) need to be explored.

• Li-S battery is the most promising high energy density (>350 Wh/kg) battery technology in the near future.